It is well known in the petroleum refining arts to catalytically crack heavy petroleum fractions, such as vacuum gas oil, or even in some cases atmospheric resid, in order to convert a substantial proportion thereof to a wide range of petroleum fractions. It is conventional to recover the product of catalytic cracking and to distill, and thereby resolve, this product into various fractions such as light gases; naphtha, including light and heavy gasoline; distillate fractions, such as heating oil and Diesel fuel; lube oil base fractions; and heavier fractions.
Where the petroleum fraction being catalytically cracked contains sulfur, the products of catalytic cracking will also likely contain sulfur impurities. In particular, it is well known that the heavy gasoline fraction is one portion of the product in which sulfur impurities seem to concentrate.
Therefore, it has been well known in the petroleum arts to subject this fraction to desulfurization processes. One such conventional, commercially known process is desulfurization by hydrotreating.
In one general type of conventional, commercial operation, the heavy gasoline is contacted with a suitable hydrotreating catalyst at elevated temperature and somewhat elevated pressure in the presence of a hydrogen atmosphere. One suitable family of catalysts which has been widely used for this service is a combination of a group VIII and a group VI element, such as cobalt and molybdenum, on a suitable substrate, such as for example alumina.
It is also well known that naphthas, often light or full range naphthas, are catalytically reformed so as to increase their octane numbers by converting at least a portion thereof to aromatics. Fractions to be fed to catalytic reforming, such as over a platinum type catalyst, for the purpose of upgrading their octane number, must also be desulfurized before reforming because the reforming catalyst is generally not sulfur tolerant. Thus, naphthas are usually pretreated to reduce their sulfur content before reforming.
Aromatics are generally the source of very high octane number, particularly very high research octane numbers. Therefore, while, on the one hand, they are quite desirable components of the gasoline pool, on the other hand, aromatics, and particularly benzene, have been the subject of severe limitations as a gasoline component because of its adverse effect upon the ecology.
To the extent that it is possible, it has become desirable to create a gasoline pool in which the higher octanes are contributed by the olefinic and branched chain paraffinic components, rather than the aromatic components. Light and full range naphthas can contribute substantial volume to the gasoline pool, but, without reforming, they do not have substantial octane to contribute.
In the hydrotreating of petroleum fractions, particularly naphthas, and most particularly heavy cracked gasoline, the molecules containing the sulfur atoms are mildly hydrocracked so as to release their sulfur, usually as hydrogen sulfide. After the hydrotreating operation is complete, the product may be fractionated, or even just flashed, to release the hydrogen sulfide and collect the now sweetened gasoline.
This is an excellent process that has been practiced on gasolines and heavier petroleum fractions for many years. It works well and produces a satisfactory product. However, it does have disadvantages.
Cracked naphtha, as it comes from the catalytic cracker and without any further treatments, such as purifying operations, has a relatively high octane number. It also has an excellent volumetric yield. As such, cracked gasoline is an excellent contributor to the gasoline pool. It contributes a large quantity of product at a high blending octane number. In some cases, this fraction may contribute as much as up to half the gasoline in the refinery pool. Therefore, it is a most desirable component of the gasoline pool, and it should not be lightly tampered with.
A substantial portion of the octane of cracked naphtha is due to the olefin content of the naphtha. Catalytic cracking is particularly adept at producing olefinic products which, in the gasoline boiling range, have very high octanes.
Other highly unsaturated fractions boiling in the gasoline boiling range, which are produced in some refineries or petrochemical plants, include pyrolysis gasoline. This is a fraction which is often produced as a by-product in the cracking of petroleum fractions to produce light unsaturates, such as ethylene and propylene. Pyrolysis gasoline has a very high octane number but is quite unstable because, in addition to the desirable olefins boiling in the gasoline boiling range, this fraction also is unstable because it contains a substantial proportion of diolefins.
Hydrotreating of any of the sulfur containing fractions which boil in the gasoline boiling range causes a reduction in the olefin content thereof, and therefore a reduction in the octane number thereof. While hydrotreating reacts hydrogen with the sulfur containing molecules in order to convert the sulfur and to remove such as hydrogen sulfide, as with any operation which reacts hydrogen with a petroleum fraction, the hydrogen does not only react with the sulfur as desired. Unfortunately, some of the hydrogen also tends to saturate at least some of the unsaturation in the molecules of the fraction being hydrotreated. Some of the hydrogen may also cause some hydrocracking as well as olefin saturation, depending on the conditions of the hydrotreating operation.
In any case, regardless of the mechanism by which it happens, hydrotreating not only removes harmful sulfur from the fraction being treated, but also lowers the octane number of that fraction. Further, as the degree of desulfurization increases, the octane number of the normally liquid gasoline boiling range product decreases. Therefore, in these days of relatively shorter supply of hydrocarbons, particularly sweet hydrocarbons, in view of the growing need to produce gasoline fuels with higher octane number, and because of current ecological considerations, that the a desire to produce cleaner burning fuels, there is a conflict between producing more and higher octane gasoline on the one hand, and producing gasoline having a lower sulfur content, which is therefore cleaner burning and less polluting to the atmosphere, on the other.
In a completely different area of petroleum refining, it is known, and it has been known for some time, that various acid acting zeolitic materials have great value in upgrading petroleum fractions. For example, commercially practiced catalytic cracking is substantially always carried out using a catalyst which comprises an acid acting zeolitic behaving refractory material as at least one of its components.
It is also well known, and widely practiced commercially, to catalytically upgrade distillate and lube oil base fractions of petroleum in order to remove waxy components therefrom and thus reduce their pour point, that is the lowest temperature at which they will still pour. This type of operation is often carried out with the aid of a dewaxing catalyst, which usually comprises as at least one of its important, active components, an intermediate pore sized zeolitic acting acidic refractory material.
The dewaxing of distillate and/or lube fractions is usually accomplished at elevated temperatures and somewhat elevated pressures, and usually in the presence of hydrogen. The usual intent is for the pressure under which the reaction is carried out, and the amount of hydrogen in the reaction zone, to be controlled such that the hydrogen acts predominantly to keep the coke make on the catalyst down, and not such that substantial hydrocracking is supported.
Actually, except for, in some cases, the amount of hydrogen and the reaction pressure, the operating conditions for processes of dewaxing of distillate and lube fractions are often quite similar to the operating conditions of a process for hydrodesulfurization by hydrotreating. The catalyst, however, is quite different.
The purpose of a hydrotreating operation is to convert the molecules containing the undesirable sulfur impurities so as to release the sulfur from the molecules as hydrogen sulfide. The purpose of a dewaxing operation is to mildly selectively crack the longer chain paraffinic and near paraffinic molecules in a higher boiling distillate or lube base fraction which are primarily responsible for the unacceptably high pour point of the fraction. This mild selective cracking converts these undesirable paraffinic molecules to lower boiling materials which are easily separated from the remaining distillate fraction, which now has a lower pour point. Suitably, dewaxing catalysts are acid acting zeolitic behaving materials which have restricted pore dimensions which will allow the ingress and egress of only selected size and shape molecules into their pore system. Since most, if not all, of the acid activity of these catalysts exists within their pore system, by limiting the access of feed molecules to these acidic cracking cites, only selected molecules of the feed are cracked.
A good dewaxing process will convert a minimum of the feed to lower boiling products. The intention and desire is to produce a product which has the highest possible yield in the distillate or lube oil boiling ranges, that is the boiling range of the feed material, while selectively removing as few as possible of those molecules which cause the pour point of the distillate or lube feed material to be higher than desired.
The operating conditions for dewaxing processes are usually selected so as to convert a minimum of the feed, consistent with the desired properties of the product. Further, since this operation is carried out under hydrogen pressure, and since at least some of the cracked product falls into the naphtha boiling range, the operating conditions are selected so as to accomplish as little olefin saturation as possible, again, consistent with the overall objective of lowering the pour point of the feed.
Suitable intermediate pore size zeolitic behaving catalytic materials are exemplified by those acid acting materials having the topology of intermediate pore size aluminosilicate zeolites. These similarly behaving zeolitic catalytic materials are exemplified by those which, in their acid form, have a Constraint Index between about 2 and 12. Reference is here made to U.S. Pat. No. 4,784,745 for a definition of Constraint Index and a description of how this value is measured. This patent also discloses a substantial number of porotectosilicate materials having the appropriate topology and the pore system structure to be useful in this service. The entirety of this patent is incorporated herein by reference.
It should be understood that these exemplary materials are particularly exemplary of the topology and pore structure of desired acid acting refractory solids. It is not intended that this patent be referred to as limiting the type of catalysts to be used for this service to aluminosilicates. Other compositions of refractory solid materials which have the desired acid activity, pore structure and topology are similarly well suited.